PROJECT SUMMARY The overall objective of this proposed research is to develop methods for the clinical assessment of lesion structure and activity. Improved imaging technologies employing non-ionizing radiation will significantly improve caries diagnosis and management. It is not sufficient to simply detect carious lesions, methods are needed to assess the activity of the lesion and determine if chemical intervention is needed. The central hypothesis underlying this proposal is that there are structural differences between active lesions due to caries, arrested lesions and developmental defects, and these differences can be quantified using optical imaging methods. There is a transparent highly mineralized outer surface zone presented on arrested lesions formed due to remineralization, which leads to decreased permeability to fluids including water and plaque generated acids. Previous studies have shown that the thickness of this surface zone can be measured nondestructively using polarization sensitive optical coherence tomography (PS-OCT), and have demonstrated that near-IR (NIR) reflectance and thermal imaging coupled with dehydration can be used to measure changes in the permeability of lesions in enamel and dentin. The overall objective of this proposal will be achieved through the following specific aims: (1) To test the hypothesis that the thickness of the highly mineralized transparent surface zone formed during remineralization correlates with the lesion permeability and activity; (2) To test the hypothesis that thermal and NIR reflectance dehydration methods can be used to measure the activity of secondary carious lesions around composite restorations; (3) To test the hypothesis that compact thermal and NIR reflectance probes for lesion activity assessment can be fabricated that are suitable for in vivo use. This project will allow us to determine the structural changes needed to arrest active lesions and provide methods for assessing the activity of lesions in a single examination to avoid unnecessary treatment and cavity preparations.